Long term scheduler determines which programs are admitted 
to the system for processing. It controls the degree of 
multiprogramming. Once admitted, a job becomes a process. 
Medium term scheduling is part of the swapping function. 
This relates to processes that are in a blocked or 
suspended state. They are swapped out of real-memory until 
they are ready to execute. The swapping-in decision is 
based on memory-management criteria.
Short term scheduler, also know as a dispatcher executes 
most frequently, and makes the finest-grained decision of 
which process should execute next. This scheduler is 
invoked whenever an event occurs. It may lead to 
interruption of one process by preemption.